Austenitic stainless steel

ABSTRACT

An austenitic stainless steel, which comprises by mass percent, C: not more than 0.10%, Si: 0.01 to 1.0%, Mn: 0.01 to 2%, Cr: 16 to 18%, Ni: more than 10% to less than 14%, Mo: more than 2.0% to not more than 3.0%, N: 0.03 to 0.10%, one or more elements selected from V, Nb and Ti satisfying the following formulas (1) and (2), 
       0.0013≦(V/51)+(Nb/93)+(Ti/48)≦0.0025  (1), 
       {(C/12)+(N/14)}−{(V/51)+(Nb/93)+(Ti/48)}≦0.0058  (2), 
     wherein each element symbol in the formulas (1) and (2) represents the content (by mass %) of the element concerned, with the balance being Fe and impurities, wherein the content of P is not more than 0.04% and the content of S is not more than 0.003% among the impurities, has excellent corrosion resistance, in particular, excellent intergranular corrosion resistance. The preferable contents of Nb and Ti are not more than 0.030% and not more than 0.050%, respectively.

This application is a continuation of the international application PCT/JP2007/059094 field on Apr. 26, 2007, the entire content of which is herein incorporated by reference.

TECHNICAL FIELD

The present invention relates to an austenitic stainless steel, having excellent corrosion resistance, in particular, excellent intergranular corrosion resistance, which can be used as structural members for a nuclear power plant, a chemical plant, or the like.

BACKGROUND ART

A SUS 316 stainless steel, which contains Mo, has been used as structural members for a nuclear power plant, a chemical plant or the like, because of excellent mechanical properties with good workability, in addition to excellent resistance to corrosion such as pitting corrosion and general corrosion, compared to those of a SUS 304 stainless steel. However, when the said SUS 316 stainless steel is welded or heated at high temperatures, sometimes a marked intergranular corrosion occurs in the heat affected zone which is produced by welding or by high temperature heating. This intergranular corrosion is called “sensitization”, and is caused by the formation of the Cr depleted zone which is poor in corrosion resistance. The above-mentioned Cr depleted zone is formed, in the welding or the heating process, by Cr carbide precipitation at the grain boundary and a decrease of the Cr concentration there. Furthermore, depending on the stress condition of the materials, intergranular stress corrosion crackings may also occur.

In the conventional measure to suppress the sensitization, a lowering of the C content and fixing of the C in the Ti and/or Nb compounds within the grains have been employed in order to prevent the formation of the Cr depleted zone through Cr carbide precipitation. However, sufficient results have not been obtained by the above-described methods.

Austenitic stainless steels with a lowered C content and/or with a small additional amount of V, Ti, Nb, and so on are, for example, disclosed in the following Patent Documents.

The Patent Document 1 (Japanese Laid-Open Patent Publication No. 55-89458) discloses an austenitic stainless steel for use in an environment of high temperature and low chlorine concentration, containing one or more elements selected from Ti, Nb, Ta, Zr and V in order to prevent a deterioration of resistance to stress corrosion cracking due to N. In this Patent Document, stress corrosion cracking due to N can be prevented by lowering the solute N within the matrix of the stainless steel by forming nitrides with Ti, Nb, Ta, Zr or V However, regarding the deterioration of resistance to stress corrosion cracking caused by the sensitization due to C, only the lowering the C content is considered.

The Patent Document 2 (Japanese Laid-Open Patent Publication No. 2003-213379) discloses an austenitic stainless steel having excellent corrosion resistance, containing Ti and/or Nb in order to suppress the Cr nitride precipitation on the grain boundaries. In this Patent Document, however, it is not considered that Ti and/or Nb can fix not only N, but also fix C within the grains. Similar effects of V on the fix of N and C in the grains are also not considered. Moreover, proper amounts for adding of Ti and Nb, which should depend on the contents of N and C, are not disclosed.

The Patent Document 3 (Japanese Laid-Open Patent Publication No. 5-59494) discloses an austenitic stainless steel excellent in suppressing irradiation assisted segregation, which contains one or more elements selected from Ti, Zr, Hf, V, Nb, and Ta. According to the said Patent Document, a point defect formation by neutron irradiation is suppressed by Ti, Zr, Hf, V, Nb, or Ta, leading to the suppression of element migration, that is, Cr from the grain boundaries, and Ni, Si, P and S to the grain boundaries. However, the role of the above-described elements on fixing C and/or N as carbo-nitrides within the grains is not considered. Furthermore, according to the said Patent Document, a large amount of Ti, Zr, Hf, V, Nb, and Ta is necessary to achieve the above-described effects.

The Patent Document 4 (Japanese Laid-Open Patent Publication No. 2005-23343) discloses an austenitic stainless steel with a fine grain structure on the surface layer, containing one or more elements selected from V, Nb, Ti and Zr. In the said steel, V, Nb, Ti and Zr are added for grain refinement. However, interaction between other elements such as C or N and above-described alloying elements is not considered.

The Patent Document 5 (Japanese Laid-Open Patent Publication No. 57-158359) discloses a corrosion resistant austenitic stainless steel, containing Ti+Nb of 0.05 to 0.10%. The said Patent Document mentions that grain boundary precipitation of carbides and/or nitrides is suppressed by the composite addition of Nb and Ta. However, when the content of Nb is more than 0.5%, pitting corrosions or macro-streak-flaws may occur.

Patent Document 1: Japanese Laid-Open Patent Publication No. 55-89458,

Patent Document 2: Japanese Laid-Open Patent Publication No. 2003-213379,

Patent Document 3: Japanese Laid-Open Patent Publication No. 5-59494,

Patent Document 4: Japanese Laid-Open Patent Publication No. 2005-23343,

Patent Document 5: Japanese Laid-Open Patent Publication No. 57-158359.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

The objective of the present invention is to provide an austenitic stainless steel having excellent corrosion resistance, in particular, excellent intergranular corrosion resistance.

Means for Solving the Problems

The basic technical idea of the present invention is to prevent intergranular corrosion due to sensitization in austenitic stainless steels, by suppressing the Cr carbo-nitrides precipitation on the grain boundaries by means of fixing C as Cr carbo-nitrides within the grains.

In order to suppress the precipitation of the Cr base carbo-nitrides on the grain boundaries in the SUS 316 type stainless steels, it is desirable that the carbo-nitride formation elements of C and N should be fixed within the grains as carbo-nitrides with V, Nb and Ti, whose affinities with C and N are much greater than with Cr. It can be seen from the aspect of the standard formation free energy that it is easier for V, Nb and Ti to form a nitride than to form a carbide.

SUS 316 type stainless steels used for structural members, usually, contain 0.03 to 0.10% N, in order to ensure the strength. However, in the case when V, Nb or Ti is added to the SUS 316 type stainless steels with high N content, nitrides of these elements are preferentially formed. Therefore, according to the conventional idea, in order to fix C as carbides within the grains for suppressing the intergranular corrosion, a large amount of V, Nb or Ti which can fix C as carbides should be added in addition to the amount to fix N as nitrides.

Or in another way, the increase of intragranular precipitates (carbo-nitrides) owing to the excessive contents of V, Nb and/or Ti may cause a deterioration of pitting corrosion resistance or may encourage production defects such as macro-streak-flaws. From these points of view, the excessive addition of V, Nb or Ti is not desirable.

Thus, the present inventors have studied the proper amount of V, Nb or Ti addition to achieve enough suppression effect on the sensitization without the above-described negative effects, and following new findings were obtained.

(a) The present inventors investigated the precipitation behaviors of the carbo-nitrides for practical steel products, and found that, even in the case when just enough of V, Nb and/or Ti for forming nitrides with N were added, the said elements also combined with C, and therefore, carbo-nitrides were also formed. This can be ascribed to the kinetic behavior in addition to the above-described equilibrium thermodynamics. That is to say, V, Nb and Ti combine not only with N by equilibrium thermodynamics but also with C.

(b) Next the present inventors investigated the resistance to the intergranular corrosion of the SUS 316 type stainless steels with various amounts of V, Nb and Ti, and found that stainless steels having excellent intergranular corrosion resistance were obtained by containing the proper amounts of V, Nb and Ti so that the following formula (1) was satisfied, and moreover, by controlling the relationship between the C and N contents and the contents of V, Nb and Ti so that the following formula (2) was satisfied.

0.0013≦(V/51)+(Nb/93)+(Ti/48)≦0.0025  (1),

{(C/12)+(N/14)}−{(V/51)+(Nb/93)+(Ti/48)}≦0.0058  (2),

wherein each element symbol in the formulas (1) and (2) represents the content (by mass %) of the element concerned.

When the value of (V/51)+(Nb/93)+(Ti/48) in the above formula (1) is less than 0.0013, the excellent intergranular corrosion resistance is not obtained. However, the increase of intergranular precipitates owing to the excessive contents of V, Nb and/or Ti is not desirable, since a large amount of intragranular precipitates may cause a deterioration of pitting corrosion or may encourage production defects such as macro-streak-flaws. Therefore, the upper limit of the value of (V/51)+(Nb/93)+(Ti/48) is set to 0.0025.

In the relationship between the C and N contents and the contents of V, Nb and Ti, when the value of the left hand member in the formula (2) exceeds 0.0058, the intergranular corrosion resistance deteriorates due to the increase of intergranular carbo-nitride precipitates.

Nb and Ti easily form carbides, because of the strong affinity with C, compared to V, and grow into intragranular precipitates, leading to a deterioration in pitting corrosion resistance. Therefore, the excessive addition of Nb and Ti should be avoided and preferable contents are less than 0.030% for Nb and 0.050% for Ti.

The present invention has been accomplished on the basis of the above-described findings. The gists of the present invention are the following austenitic stainless steels.

(1) An austenitic stainless steel, which comprises by mass percent, C: not more than 0.10%, Si: 0.01 to 1.0%, Mn: 0.01 to 2%, Cr: 16 to 18%, Ni: more than 10% to less than 14%, Mo: more than 2.0% to not more than 3.0%, N: 0.03 to 0.10%, one or more elements selected from V, Nb and Ti satisfying the following formulas (1) and (2), with the balance being Fe and impurities, wherein the content of P is not more than 0.04% and the content of S is not more than 0.003% among the impurities.

0.0013≦(V/51)+(Nb/93)+(Ti/48)≦0.0025  (1),

{(C/12)+(N/14)}−{(V/51)+(Nb/93)+(Ti/48)}≦0.0058  (2),

wherein each element symbol in the formulas (1) and (2) represents the content (by mass %) of the element concerned.

(2) The austenitic stainless steel according to above (1), wherein the content of Nb is not more than 0.030% or the content of Ti is not more than 0.050%, or, the content of Nb is not more than 0.030% and the content of Ti is not more than 0.050%, by mass percent.

EFFECT OF THE INVENTION

An austenitic stainless steel in the present invention has excellent corrosion resistance, in particular, excellent intergranular corrosion resistance. Therefore, it is very suitable to be used as a structural member where an intergranular corrosion may occur.

BEST MODE FOR CARRYING OUT THE INVENTION

The reasons for restricting the chemical compositions of the austenitic stainless steel in the present invention will next be explained. In the following description, the symbol “%” for the content of each component represents “% by mass”.

C: not more than 0.10%

C is used for deoxidation and ensuring the strength of steels. However, in order to suppress the carbide precipitation, from the view point of ensuring the corrosion resistance, it is preferable to make the C content as low as possible. Therefore, the upper limit of the C content is set to 0.10%. A more preferable content of C is not more than 0.05%. On the other hand, in order to ensure the strength as structural members, the C content is preferably not less than 0.01%. More preferably, the content of C is not less than 0.015%.

Si: 0.01 to 1.0%

Si is used for deoxidation of steels. In the steel of the present invention, the content of Si is set to not less than 0.01%. Since excessive Si content encourages forming inclusions, it is preferable to make the Si content as low as possible. Therefore, the content of Si is set to 0.01 to 1.0%.

Mn: 0.01 to 2%

Mn is effective in deoxidizing the steel and stabilizing the austenitic phase. The said effects are obtained if the content of Mn is not less than 0.01%. On the other hand, Mn forms sulfides with S, and the said sulfides exist as non-metallic inclusions in the steel. Moreover, when the steel products are welded, Mn preferentially condenses the surface of the welds and therefore brings out a deterioration of corrosion resistance in the steel products. Therefore, the proper Mn content is set to 0.01 to 2%.

Cr: 16 to 18%

Cr is an indispensable element in order to ensure the corrosion resistance of steels. A sufficient corrosion resistance is not obtained, when the content of Cr is less than 16%. In the present invention, a Cr content of not more than 18% is sufficient. A Cr content exceeding 18%, leads to the deterioration of workability and also increases the cost of steels for practical use. Moreover, it makes difficult to keep the austenitic phase stable. Therefore, the upper limit of the Cr content is set to 18%. The content of Cr is more preferably not more than 17.5%.

Ni: more than 10% to less than 14%

Ni is an important element for the stabilization of the austenitic phase and maintains the corrosion resistance. From the view point of corrosion resistance, the Ni content of more than 10% is necessary. From a view point of weldability, the upper limit of the Ni content has a relationship with the content of Cr, and is set to less than 14%. The lower limit of the Ni content is more preferably 10.5%, and the upper limit thereof is more preferably 13%.

Mo: more than 2.0% to less than 3.0%

Mo has an effect for stabilizing the passive film, and is an indispensable element to ensure the pitting corrosion resistance and/or general corrosion resistance. However, when Mo precipitates as intermetallic compounds with Fe, Ni, Cr and the like on the grain boundaries, the intergranular corrosion resistance is deteriorated. Therefore, the content of Mo, which ensures the general corrosion resistance without deteriorating the intergranular corrosion resistance, is set to more than 2.0% to less than 3.0%. The more preferable upper limit content of Mo is 2.5%.

N: 0.03 to 0.10%

The content of N is set to not less than 0.03%, in order to ensure the strength of steels. However, N forms nitrides with Cr in the steel, and the said nitrides cause a deterioration of intergranular corrosion resistance. Thus, the content of N is set to not more than 0.10%. The lower limit of the N content is more preferably 0.04%, and the upper limit thereof is more preferably 0.08%.

V, Ti and Nb: one or more elements selected from these, within a range satisfying the above-mentioned formulas (1) and (2)

By the reasons described in paragraphs [0021] and [0022], the contents of V, Ti and Nb are set within the range satisfying the formulas (1) and (2). Also, by the reason described in paragraph [0023], the preferable contents of Nb and Ti are set to not more than 0.030% and not more than 0.050%, respectively.

The stainless steel according to the present invention comprises the components described above with the balance being Fe and impurities. It is necessary, however, to suppress the contents of P and S among the impurities in the following manner.

P: not more than 0.04%

Since the increase of P content deteriorates the corrosion resistance, the content of P is preferable as low as possible. Therefore, the upper limit of the P content is set to 0.04%.

S: not more than 0.003%

forms sulfides, which are non-metallic inclusions. Also, S is an element which deteriorates the hot workability. Therefore, the content of S is preferable as low as possible. Consequently, the upper limit of the S content is set to 0.003%.

EXAMPLE

Stainless steels having chemical compositions shown in Table 1 were melted and cast to form ingots. The ingots were hot-forged and hot-rolled into 6 mm thick plates, and then the said hot rolled plates with 6 mm thickness were cold rolled into 4 mm thick plates. The cold rolled plates were subjected to solution treatment, namely maintained at 1060° C. for 15 minutes and water cooled. Then they were subjected to a sensitizing treatment, that is to say, they were heated at 650° C. for 2 hours and cooled in air. After the said sensitizing treatment, the corrosion rate was measured according to the Method of Ferric Sulfate-Sulfuric Acid Test (JIS G 0572), which is a typical test method for evaluating the intergranular corrosion resistance. The value of “(V/51)+(Nb/93)+(Ti/48)” in the formula (1) and the value of left side member of the formula (2) are also shown in Table 1.

[Table 1]

TABLE 1 Test Chemical Composition (mass %, Balance: Fe) Value of Value of Division No. C Si Mn P S Cr Ni Mo V Ti Nb N formula (1) formula (2) Examples of the 1 0.037 0.35 1.39 0.025 0.0003 16.21 11.22 2.19 0.091 0.002 0.021 0.0555 0.0021 0.0050 present 2 0.042 0.36 1.46 0.028 0.0007 16.92 11.20 2.09 0.082 0.004 0.012 0.0516 0.0018 0.0054 invention 3 0.036 0.34 1.59 0.027 0.0005 16.07 11.33 2.03 0.065 0.003 0.011 0.0509 0.0015 0.0052 4 0.045 0.41 1.51 0.026 0.0006 16.73 11.16 2.07 0.077 0.038 0.006 0.0452 0.0024 0.0046 5 0.046 0.47 1.47 0.025 0.0004 17.16 11.89 2.21 0.063 0.012 0.007 0.0428 0.0016 0.0053 Comparative 6 0.040 0.36 1.37 0.021 0.0005 16.36 11.23 2.05 0.060 0.003 0.002 0.0542 0.0013 0.0059 examples 7 0.039 0.39 1.39 0.025 0.0008 16.42 11.16 2.11 0.050 0.002 0.003 0.0489 0.0011 0.0057 Note 1: Value of formula (1): (V/51) + (Nb/93) + (Ti/48) Note 2: Value of left hand member of formula (2): {(C/12) + (N/14)} − {(V/51) + (Nb/93) + (Ti/48)} Note 3: Underlined numbers mean out of the range regulated by the present invention.

The test and evaluation results of intergranular corrosion resistance are shown in Table 2. In the tests of intergranular corrosion resistance, the scatter of the two specimens was negligibly small in the examples of the present invention. On the other hand, in the comparative examples, since the scatter of the two specimens was very large, an additional 4 specimens were tested and so a total of 6 specimens were evaluated. The large scatter of the data in the comparative examples originated from falling-off of surface grains during testing due to the poor intergranular corrosion resistance. The intergranular corrosion resistance was evaluated by “◯”, when corrosion rates of all the specimens were less than 3 g/m²h, and was evaluated by “x”, when at least one result of the plural specimens exceeded 3 g/m²h.

[Table 2]

TABLE 2 Test Division No. Corrosion rate (g/m²h) Evaluation Examples of the 1 0.81, 0.93 ∘ present 2 0.93, 0.98 ∘ invention 3 0.79, 0.80 ∘ 4 0.78, 0.77 ∘ 5 1.05, 1.20 ∘ Comparative 6 1.42, 1.65, 2.32, 2.87, 3.72, 5.14 x examples 7 3.05, 3.64, 5.75, 6.35, 6.84, 8.00 x

As is apparent from Table 2, the corrosion rates of Nos. 1 to 5 in the examples of the present invention were low, that is, they had excellent intergranular corrosion resistance. On the other hand, in the comparative examples No. 6 and No. 7, the steel had a chemical composition outside the value of the formula (1) or (2) specified in accordance with the present invention. Consequently, their intergranular corrosion resistance was poor.

Although only some exemplary embodiments of the present invention have been described in detail above, those skilled in the art will readily appreciated that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of the present invention. Accordingly, all such modifications are intended to be included within the scope of the present invention.

INDUSTRIAL APPLICABILITY

According to the present invention, an austenitic stainless steel having excellent intergranular corrosion resistance, and so having both excellent pitting corrosion and general corrosion resistance can be provided. This stainless steel can show excellent effects, when they are used as structural members for a nuclear power plant, a chemical plant, or the like. 

1. An austenitic stainless steel, which comprises by mass percent, C: not more than 0.10%, Si: 0.01 to 1.0%, Mn: 0.01 to 2%, Cr: 16 to 18%, Ni: more than 10% to less than 14%, Mo: more than 2.0% to not more than 3.0%, N: 0.03 to 0.10%, one or more elements selected from V, Nb and Ti satisfying the following formulas (1) and (2), with the balance being Fe and impurities, wherein the content of P is not more than 0.04% and the content of S is not more than 0.003% among the impurities: 0.0013≦(V/51)+(Nb/93)+(Ti/48)≦0.0025  (1), {(C/12)+(N/14)}−{(V/51)+(Nb/93)+(Ti/48)}≦0.0058  (2), wherein each element symbol in the formulas (1) and (2) represents the content (by mass %) of the element concerned.
 2. The austenitic stainless steel according to claim 1, wherein the content of Nb is not more than 0.030% or the content of Ti is not more than 0.050%, or, the content of Nb is not more than 0.030% and the content of Ti is not more than 0.050%, by mass percent. 